V cone type flow (VNZ meter)
Published on：2015-1-14 9:15:10 Hits：
1. Preamble (Overview)
With plates, nozzles and venturi represented differential pressure flowmeter has been successfully used in industry more years. On the basis of the accumulation of a large number of practical experience, the latest international standard ISO5167-1 on standardized differential pressure flowmeter, ISO5167-2, ISO5167-3 and ISO5167-4 was officially announced by the International Organization for Standardization (ISO) 2003 年 3 月 carried out. The so-called "standardized" test calibration is not required to determine the relationship between the differential pressure and flow, and can estimate the measurement error. Present in all flow meter only this meter is the only standard to achieve this flow meter.
Because orifice flowmeter has been standardized and simple structure, strong; easy to copy, versatile, inexpensive to obtain a wide range of large-scale application. However, due to defects in its own orifice flowmeter structure also has some major drawbacks;,, such as unstable discharge coefficient of linear difference, repeatability is not high, due to the many factors that affect the accuracy is not high, the contamination easy easy is worn, pressure loss larger than the small-scale (range degrees), high field installation conditions, the required straight pipe is too long.
Optimized for generating differential throttling device improvements has not been interrupted, the size of its geometric forms, cutting pieces of the parameters, the entrance edge profile, taking pressure and throttle approach can replace the orifice of the development work has been going . For example: In order to measure the flow of dirty fluid has developed a large non-standard throttling device to include: round orifice, eccentric orifice, wedge plates, wear plates, circular plates, elbow (elbow) and so on. This improvement, improved work until the mid-1980s, before the development of a qualitative leap, namely: the fluid throttle shrink to the central axis of the pipeline near to fundamentally change the concept of the use of coaxially mounted in the pipeline will be V-shaped tip cone fluid throttle gradually shrink into the side wall of the pipe. By measuring the differential pressure within the V-shaped cone is measured before and after the flow rate. Flowmeter for differential pressure flowmeter opened a new chapter within this V-shaped cone (VNZ). After 10 years of repeated testing and application, currently it has been generally understood it and accept it as a more efficient flow meter. Proved: use VNZ flowmeters at shorter straight pipe conditions to a wider range than on clean or dirty fluids achieve more accurate and efficient flow measurement. This chapter will demonstrate how it works in detail, the structure, the composition of the whole system; the advantages and disadvantages, the typical product analysis and typical application examples and experience; the main technical performance indicators.
2.1 The basic principle of differential pressure flowmeter
Overall, the work of the differential pressure flowmeter based on the fact: If the fluid flow through a contraction (throttling) parts, fluid will be accelerated. This will enable it to accelerate the kinetic energy of the fluid increases, while in accordance with the energy conservation law, in which the fluid is accelerated at the static pressure will decrease a corresponding value. Conservation of energy tells us: In a closed system, the total energy of the fluid is a constant. For further quantitative analysis, see the following Figure 1
In a cross-sectional average velocity of the fluid is V1, a density ρ1, pipe cross-sectional area in a cross section is A1; when the fluid flows through the cross-section 2, corresponding to an average flow rate V2, a density ρ2 cross-sectional area is A2, based on the principle of continuity of fluid flow has the following relationship: V1 · A1 · ρ1 = V2 · A2 · ρ2 .................. (1)
If the fluid is a liquid, can be considered prior to shrink after its constant density, i.e.
ρ1 = ρ2 = ρ
So the volume of liquid flow: qV = V1 · A1 = V2 · A2 ..................... (2)
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